Method of cleaning and/or annealing metal



Patented Mar. 5, 1929.

UNITED STATES NOAK VICTOR HYLBINETTE, 01 NEW YORK, N. Y.

METHOD OF CLEANING AND/OR ANNEAI ZINQ METAL.

No Drawing.

This invention relates to.improve1nents in the cleaning of metalsurfaces and more particularly the cleaning of metal surfaces such asnickel,'nickel alloys, etc. from insoluble or 5 diificultlysolubleoxides.

The invention also relates to improvements in the annealing ofsuch-metals with protection of the annealed metals from surfaceoxidation during cooling. When the surfaces of the metal' objects to beannealed require cleaning from an oxide film or coating, the

cleaning and annealing operations can ad vantageously be combined in asingle operation.

Acid resisting and heat resisting metals, such as nickel and alloysconsisting mainly of nickel, have the peculiar property of becomingcovered by an insoluble and strongly adhering coating of oxide whenheated in an oxidizing atmosphere. This property, valuable in manyinstances, is a great drawback in the mechanical Working of the metals,particularly in thecase of the working of the metals by rolling.

In rolling mill work, the present practise consists in cleaning thesurface of the metal by mechanical means, such as turning, planing,scraping or sand blasting, after the hot rolling has been stopped at athickness appreciably greater than that of the final article, and inthen continuing with cold rolling and intermediate annealing in anon-oxidizing atmosphere (in annealing boxes or otherwise). Suchmechanical cleaning of the metal surfaces is expensive andunsatisfactory.

The present invention provides a method for removing such oxide coatingin a way that allows the hot Working to be carried considerably furtherthan heretofore, and allows the cleaning and annealing to be done muchbetter and quicker than heretofore.

According to the present invention the metal, preferably in a heatedstate, is immersed in a molten bath of a slag capable of dissolving themetal oxides, such as nickel oxide, and the metal is left in the moltenbath a sufficient length of time at a sufficiently high temperature todissolve such oxide or oxides. In the case of nickel or nickel alloys,and with hot metal, only a few minutes are required at a temperature ofabout 800 C. of

the metal and of the slag. The metal is them withdrawn from the moltenbath, and it is protected against reoxidizing by a thin coat-Application filed July 29,

that it can be completely removed after cool- 1921. Serial No. 488,468.

ing of the slag which adheres thereto. The metal is finally cooled andthe layer of slag is removed;

The term slag,as used in this specification, means any substance ormixture of substances which when fused is capable of dissolving metallicoxides.

Inasmuch as the molten slag is to be used for dlssolving the metal oxidecoating or film, it should be of a suitable composition for thispurpose, The slag or flux should alsobe of such a composition as to beeasily removed from .the metal after cooling, but sufliciently adheringto protect the metal surface after removal from theslag bath and durmgcooling. Some silica may be present in the slag, but if the silicacontent is too high the slag will stick to the metal like an enamel andmay be difficult to remove, and, upon cooling, such slag, if notcompletely removed, 7 is liable to berolled into the metal. An excess ofsilica in the slag should therefore 111 most cases be avoided. It isimportant that the slag should be of such a character ing. It isdesirable therefore that the slag should be soluble in water, or atleast partly soluble inwater, so that it can be completely removed bydissolving without the use of acids. Borax alone, or a mixture of soda(soda-ash) and borax, can be used, but it is better to use a mixturecontaining, for example, borax, soda, sodium sulphate, and sodiumsilicate in about equal proportion. For best results it is desirablethat the slag should be so compounded, that it will stick to the metalsurface after it has been cleaned and/or annealed and protected frombeing oxidized while cooling in the air, and that the slag should comeofffairly easily after the object has cooled below the point at whichthe metal will discolor and oxidize in the air. it is evident that ifthe metal oxides are removed and the metal surfaces cleaned, forexample, at a red heat; and if the metal is lifted out of the bath atthat temperature and without protection from the atmosphere it willagain become oxidized on the surface.

It is impractical to cool the metal surface while still immersed sincethis would involve cooling the molten bath each timeit is used.Accordingto the present invention, the slag is of such a character thatit covers the metal surface and protects it at elevated temperatures sothat the metal can be cooled without oxidizing. The slag shouldtherefore be noncracking, and it may even be of a plastic or stickyconsistency, at the elevated temperatures atwhich the metalwould'oxidize if not protected. If the slag has too high a melting pointand begins to crack off at too high a temperature, the metal will beexposed and oxidize. If the protecting layer of slag does not crack of]?at all, even at low temperatures,

below those at which the metal requires prooff fairly. completely, sothat the bulk of the slag will be obtained in a solid state such that itcan be remelted without further manipulation and so that only asmallamount has to be removed from the metal by water or steam or weakacid. It is fairly easy to obtain a slag of this character and a widerange in its composition is permissible.

It is desirable that the slag used shall contain nothing of a corrosivenature which will attack the metallic surface. Such fluxes aschloride-s, fluorides and bi-sulphates, accordingly, are not to berecommended, and, when used should be present only in relatively smallquantities such that they have no objectionable corroding action. If anyconsiderable corrosion takes place, there is an unnecessary loss ofmetal, and-the operation may become objectionable due to gas evolution,etc., while the resulting metallic surface tends to become etched orcorroded and therefore less desirable, for many purposes than when theoxide film alone is removed without such corrosion. Small quantities ofthe above mentioned ingredients may, however, be used, althoughgenerally they should not be used in objectionable amount. i

In thespecific composition of the slag above mentioned, the addition ofsoda to the borax increases the-toughness and decreases the fiuidity,since borax alone is too fluid at temperatures around 700 C. Theaddition of silica, in the form of sodium silicate, aids in regulatingthe brittleness and cracking qualities of the slag. The sodium sulphateserves to dilute the other ingredients and increases the watersolubility of the slag. If the amountof silica is increased, forexample, if too much silica is inadvertently added, the slag may adhereso firmly to the metal and be so difficultly soluble in water thatdilute sulphuric acid may be reuired to remove the slag from the metalsur- 'ace. Other additions such as fluorspar, etc. are alsopossible; infact, the composltlon of the slagcan be varied between rather widelimits.

Inasmuch as the slags, which are separated .fIOIIl the cool metalsurfaces will contain the oxides dissolved from the metal surfaces, and

inasmuch as the amount of such oxides will increase with repeated use ofthe slag, it may become necessary to remove such oxides in order toprevent the impairing of the dissolving qualities of the slag andprevent an undesirable increase in its melting point or in its degree ofinsolubility, since an increase in the content of such oxides willincrease the w insolubility and also the melting point of the slag. Theslag can be regenerated or freed from such dissolved oxides, whennecessary, by cooling the slag and dissolving it in water,

precipitating the oxides, filtering the result ing solution andevaporating the solution to obtain the slag ingredients for reuse.molten bath can also be freed from oxides or regenerated by subjectingthe fused bath to electrolysis, the oxides being dissociated and themetal recovered on the cathode much as in the electrolytic production ofmetallic aluminum from a fused bath. If, however, copper is present inthe nickel alloy to be cleaned and in the oxide to be removed, then partof the copper will cement out on the.

metallic surface and will have to be removed by other means. Thisprocess therefore does not readily lend itself to the cleaning of nickelalloys containing much copper.

The process of the present invention is of more or less generalapplication, not only to nickel and nickel alloys, but also for thecleaning of other metals, although it is of particular value forcleaning surfaces having oxides which are substantially insoluble inacid pickling liquors. The process can thus be used for cleaning metalsurfacesprior to electroplating. It can also be advantageously employedfor annealing purposes, both when the metal surface does not requirecleaning from oxides, and when the metal is to be both cleaned andannealed. In the latter case the process enables the cleaning andannealing to be affected in one process. This can be accomplished bymaintaining the bath at the annealing temperature and permitting The themetal object to remain therein a sufficient period of time to bringabout the annealing subjected to rolling and while it'is still at anelevated temperature.

The apparatus required for carrying out the present process is simple incharacter. It may consist merely of a container of some refractorymaterial provided with suitable heating means either for heating thecontainer from the outside or for heating the slag from above, or forheating the' bath in both of these ways. The receptacle may be a.shallow pan or receptacle similar to a reverberatory furnaceandprovided with heat ing means for heating the slag from above; or thereceptacle may be deeper and be ex.- ternally heated. The necess aryheat may be generated by direct coal or oil furnaces or by electricheating, preferably through resistance wires or resistors submerged inthe slag.

A particularly advantageous container for carrying out the process isone made out of.

heat resisting metal such as an alloy of nickel, for example, anickel-chromium-iron alloy. The furnace construction should preferablybe such that the flame from the fuel (when a combustion furnace is used)comes in contact with the sides of the metal container but not with thetop of the container or the moltenslag therein, so that the metal to beannealed and/or cleaned does not have to pass through the flame. Whensuch a container is used, the object to be cleaned and/or annealed maybe supported from outside the furnace or may be completely immersed inthe container.

- r In the case of rods, sheets, wires, etc., the cleaning and/orannealing process can be carried out in a continuous manner by passingthe metal through the bath and regulating the time of submergencethereinso that e the desired cleaning and/or annealing will take place. If themetal is still hot from a rolling or other operation it will notmaterially cool the molten slag bath. If the metal is cold it canadvantageously be preheated before it is introduced into the moltenbath. Even in the case of individual objects which are to be cleanedand/or annealed, the metal is preferably preheated, by an open fire orin a Inufile, as this saves considerable time in the completion of theoperation, and the same tained, both the cleaning and annealingoperations are almost instantaneous.

. The cleaning and/or annealing of the metal, for example, ofnickel ornickel alloys, can advantageously be combined with the mechanicalworking of the metal, so that the further mechanical working of themetal after cleaning and/or annealing will have cleanmetal surfacesprovided therefor. Accordingly, the mechanical working of such metals ismaterially facilitated. For example, malleable nickel slabs or bars canbe rolledinto sheets or shapes of substantially the desired thickness,or dimension, by hot rolling, the resulting object or material, whilestill hot can then be cleaned from oxide, and annealed if desired, and,after the removal of the slag, can then be further worked, for example,by cold rolling to give the final desired shape or thickness. Nickel ornickel alloy sheets can thus be hot rolled to practically the desiredfinal thickness, the sheets then cleaned in the manner above described,

and finally cold rolled to the final thickness.

The cleaning operation above described leaves the metal surfaces welladapted for further manipulation, so that the finished sheets or shapesof the desired dimensions can readily be obtained by cold working of thecleaned, or cleaned and annealed, mate rial.

Even for'annealing purposes where no removal of scale,'but onlyprotection against gases, is desired, the annealing of the metal in themannerabove described is more advantageous than box-annealing inasmuchas the surface is made perfectly free from oxide and no objectionablehardening of the surface takes place, while the process is also onewhich requires only a short periodpf time and can be carried out atmoderate expense. Wire, such as nickel wire or chrome-nickel wire, canthus be annealed in the manner above described and with advantages suchas those indicated. for instance, by passing the wire through slag bathsbetween the successive dies. I

It will accordingly be seen that the present invention is of particularadvantage not only for the cleaning of metal surfaces such as thosehaving difiicultly removable oxide films, but also for the annealingof'metals,

and for the combined cleaning and annealmg of metals; while it canadvantageously be combined with the mechanical working of the metals sothat the metals can be Worked hot to practically their final shape, thencleaned and/or annealed, and finally finished by cold working.

I claim v 1. The method of cleaning metal surfaces of a slag capable ofdissolving such oxides,

.by removing oxides therefrom, which comprises submerging the same in amolten bath withdrawing the hot metal from the slag and protecting thesame against oxidation by the adhering layer of the slag, such slagcontaining only sufiicient silica to impart to the slag a stickyconsistency at the temperatures of about 300 to 700 C.

2. The cleaning of metallic nickel, nickel alloys, metals containingchromium and the like from a relatively insoluble coating of oxide,which comprises subjecting the same to the action of a moltenslagcapable of dissolving such oxide such slag containing boricacidtogether with other ingredients which give a slag which is elastic whilehot bu brittle When cold. I

3. The cleaning ofmetallic nickeh nickel alloys, metals containingchromium and the.

like, from a relatively insoluble and strongly adhering coating ofoxide, which comprises subjecting the same to the action of a moltenslag capable of dissolving such oxide, such slag containingboric acidand silica.

In testimony whereof I affix my signature.

NOAK VICTOR HYBlNETTE.

